1. Field of the Invention
This invention relates to gas turbine engines and more particularly to apparatus for controlling the flow of cooling air to the turbine section during operation of the engine.
2. Description of the Prior Art
In a gas turbine engine of the type referred to above, pressurized air and fuel are burned in a combustion chamber to add thermal energy to the gases flowing therethrough. The effluent from the chamber comprises the working medium gases and is flowed axially downstream in an annular flow path through the turbine section of the engine. A first row of nozzle guide vanes at the inlet to the turbine directs the medium gases onto a row of blades which extends radially outward from the engine rotor. An annular shroud which is supported by the turbine case surrounds the tips of the blades to minimize the leakage of working medium gases across the blade tips. In many engines the shroud is segmented and is positioned with respect to the blade tips by the turbine case.
The turbine blades are sensitive to the temperature of the medium gases flowing thereacross. As the gas temperatures increase, the blades instantaneously respond by expanding in the span-wise direction radially outward toward the shroud. The turbine case, which supports the shroud, however, is isolated from the medium gases in the flow path and is much slower to respond to the changing flow path conditions. Substantial clearance between the blade tips and the shroud is provided in the cold condition to prevent the destructive impact of the blades on the shroud as the engine is accelerated and the flow path temperatures increase. Before thermally stable conditions are reached the turbine case and the shroud supported therefrom grow radially away from the blade tips leaving again a substantial clearance which approximates the clearance in the cold condition. In order to reduce the clearance at thermally stable conditions, including maximum power and cruise, recent constructions have provided cooling air along the turbine case. Cooling the turbine case diminishes the growth of the case away from the fully expanded blades as thermal equilibrium is reached. Accordingly, the equilibrium diameter of the shroud more closely matches the diameter circumscribed by the rotating blades and the clearance is reduced.
The term "pinch point" is defined as the condition of closest proximity between the blade tips and the surrounding shroud. The clearance at the pinch point is generally positive, although in some constructions a moderate interference fit between the shroud and the passing blade tips is allowed. As is demonstrably illustrated in FIG. 7 graph, the interference or clearance is set at the pinch point by increasing or decreasing the magnitude of the initial clearance to adjust the appropriate shroud curve upward or downward. The more closely the shroud radius is tailored to the blade tip radius the smaller the magnitude of the clearance at thermally stable conditions becomes.
Apparatus for controlling the flow of turbine cooling air to achieve an optimum relationship between the clearance at stable conditions and adequate pinch point clearance comprises a significant portion of the inventive concepts disclosed and is discussed later herein. In some constructions having apparatus which is apparently similar to that of the present invention, the quantity of air flowing to the turbine vanes is controlled by a thermally responding valve. In U.S. Pat. No. 3,736,069 to Beam et al entitled "Turbine Stator Cooling Control," an annular ring having a comparatively high coefficient of thermal expansion is disposed radially outward but in intimate contact with the structure supporting the turbine vanes. As the turbine temperatures increase during the operation of the engine, the annular ring grows radially outward at a rate exceeding that of the vane supporting structure. A radial gap opens between the ring and the supporting structure to provide a path for cooling air which is then flowable through the gap to the turbine vanes.
Although the device of Beam et al appears suitable for use in controlling the flow of cooling air to the turbine case, the apparent immediate response to the valve to increasing temperatures limits its effectiveness for case temperature control.